The present invention is directed toward the field of satellite constellations. In particular, a novel inter-satellite communication method for connecting satellites is described for use with satellite constellations having overlapping orbital planes.
In the past, satellites have generally been placed in a geo-stationary orbit 22,300 miles above the Earth. Each geo-stationary satellite operated as an independent entity with user communications, data, telemetry, etc. flowing from an Earth station to the satellite and then back to the ground from the same stationary satellite. These types of satellites were fixed with respect to a particular field-of-view of the Earth, and because the satellite was located at a great distance from the Earth, it could "see" a substantial portion of the planet, such as the entire North American continent. Thus, only a few spacecraft were required for global coverage. In some satellite systems, several satellites parked in a geo-stationary orbit could also communicate with each other via an inter-satellite link ("ISL").
Although providing the advantage of being able to see a large portion of the Earth, these geo-stationary satellites and systems suffered from many disadvantages, particularly with respect to real-time communication systems, such as telephone calls, video conferencing, and real-time data transmission, including: (1) the cost to launch the relatively large satellites into the geostationary orbit was high; (2) if one satellite failed, the entire system was largely nonoperative; and (3) the time delay associated with transmission up and down from Earth to the satellite and then back was not appropriate for some types of applications.
More recently, larger constellations of non-stationary satellites have been proposed, and to date, one commercial system, Iridium, is in the process of being launched and operated. Common features of existing and planned satellite constellations include: (a) the spacecraft continually move with respect to the earth; (b) the spacecraft are positioned at a much lower altitude than the geo-stationary systems, typically located in low-earth orbit (LEO) or medium-earth orbit (MEO); (c) a plurality of orbital planes are provided, wherein an orbital plane is a set of satellites that each follow (nominally) the same orbital track or path over the Earth; and (d) user communications flow between the satellites, both within a particular plane, and between adjacent planes, using ISLs, thus forming a network in space.
The plurality of orbital planes are generally not stationary with respect to the Earth, and generally have orbital tracks that intersect with each other as they pass over the Earth. In regions where inter-satellite connections are required between spacecraft in planes moving in similar directions, inter-satellite links can be formed and held for significant periods of time since link distances and angular relationships will change slowly. Such connections are easy to manage.
However, as satellite constellations increase in size (number of satellites and planes) and complexity, the use of more complex orbital schemes will become more prevalent, such as the use of inclined orbit planes. The use of such orbits can result in regions where satellite tracks of different planes cross over each other, i.e. the orbits overlap. If the overlap occurs in regions where it is necessary to maintain the inter-satellite links, the management of inter-satellite links and the complexity of ISL terminals will be driven by the system and method for making the connections in such regions. In some situations the inter-satellite links between overlapping planes must be made and broken multiple times as linked satellites move out of the range or angular coverage capability of the ISL terminals on board each satellite.
Additionally, arrangements of orbit planes which are not stationary with respect to the Earth can create regions where satellites in certain planes are moving generally in one direction, such as towards the Southern Pole, whereas satellites in other planes are generally moving in the opposite direction. In these situations the inter-satellite links between opposing direction planes must be made and broken many times as linked satellites move out of the range of the ISL terminals on board each satellite.
In a system where dynamically changing inter-satellite connectivity is required during part of an orbit to maintain network inter-connectivity, for example between spacecraft planes moving in opposing directions, the changing connectivity region is referred to as a "seam." The problems imposed by such a region are particularly difficult to manage when both overlapping and opposing directions occur simultaneously in a large constellation of satellites, resulting in a situation where satellites are moving at different speeds relative to each other and are required to link to a plurality of other satellites simultaneously in order to form the network in space. Prior systems fail to solve the problem of how to maintain connectivity of the network in these regions.
Therefore, there remains a need in this art for a system and method of managing the inter-satellite connections between satellites in a constellation having a plurality of overlapping planes and requiring inter-satellite links in the overlap region.
There remains a further need for such a system and method of managing the inter-satellite connections between satellites with overlapping planes where satellites are moving in opposing directions.
There remains an additional need for such a system and method which limits the cost and complexity of the ISL terminals on board each satellite, minimizes the number of links that are dynamically made and broken as satellites travel about the orbital planes, minimizes the travel path (and therefore the system delay) of data flowing through the regions of the network where the orbital planes are overlapped and also minimizes the variation in delay between traffic flowing in overlapped and traffic flowing in non-overlapped regions of the network.